Part 1 example: he comes to burn your ass at the stake (worst case secenario). Hubby finds out and divorces her, you are sued for ruining her marriage (I don't care if it's consenual, how do you prove that? Get a lawyer to fight = $$$$). You are outed to your employer, your family, your friends and your sense of charater is potentially beyong repair as the dirty other guy who doesn't have properly aligned morals aka defemation of your character to everyone who matters to you.
Part 2: No, it's not the law it's a matter of ehtics or morals. These are socially accepted actions and behaviours as inherently good or right by society, these morals don't line up exclusively with the law.